An organic electroluminescent element (appropriately referred to hereinbelow as an “organic EL element”) includes a light-emitting layer disposed between multiple layers of electrodes thereby to electrically obtain luminescence, and has characteristics such as high luminous efficiency, low voltage drive, low weight, and low cost. Taking advantage of such characteristics, there have been made studies for using the organic EL element as a light source of surface light source devices such as flat-type lighting systems and back lights for liquid crystal display devices.
When the organic EL element is utilized as a light source of such surface light source devices, there is a demand for extracting light in a useful manner from the element with high efficiency. For example, although the light-emitting layer of the organic EL element itself has high luminous efficiency, the amount of light tends to be decreased by interference or the like in the layers during the passage of the light through the layered structure constituting the surface light source device until the light exits. Therefore, there is a demand for reducing such loss of light as much as possible.
As a technique for increasing the light extraction efficiency, there is known a technique of disposing a variety of optical functional layers on the surface on the light exit surface side of the organic EL element. Examples of such optical functional layers may include a structural layer having a surface that includes a plurality of concave portions or convex portions. As a specific example, Patent Literature 1 discloses a surface light source device in which a structural layer having a plurality of concave portions is disposed, via an adhesive layer, on the surface of a glass substrate which is arranged in a portion closest to a light exit surface of an organic EL element.